I fully support anon’s idea of her being captured and enslaved. The rest…. Not as much on board.
I prefer the idea that she escaped, got caught up with the wrong crowd for a while, perhaps some raiders. But when she was commanded to kill a pony slave, she rebelled.
It’s now her life’s work to free other enslaved ponies.Perhaps she’s even built up her own band/herd and they make a living stealing from slavers and other unsavory types.